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Cosmic Misfits Elude Theories

TOKYO--Astronomers have become increasingly perplexed over the last few years by a strange new class of celestial body. Too small to fit conventional definitions of brown dwarfs, they nonetheless move through star-forming regions in a manner that separates them from planets orbiting a star. Once seen as anomalies, their growing numbers are forcing astronomers to sit up and take notice (Science, 6 October 2000, p. 26). On 14 February a Japanese team raised the stakes by reporting its discovery of more than 100 of these objects in a star-forming region known as S106. "This poses a big challenge for the standard picture of star formation," says Shu-ichiro Inutsuka, a theorist at Kyoto University.

Yumiko Oasa of the University of Tokyo and colleagues there and at the National Astronomical Observatory of Japan (NAOJ) spotted the band of cosmic misfits while using NAOJ's Subaru Telescope on Mauna Kea, Hawaii. Plugging data on luminosity and estimated age into models of how very low-mass stars evolve, the team estimated the objects' masses at 5 to 10 times that of the planet Jupiter. An analysis of their infrared emissions placed the objects within the region. "Our discovery sheds new light on the ubiquity of isolated planetary mass objects," says Oasa about her work, the basis for a Ph.D. thesis approved last month.

Joan Najita, an astronomer at the National Optical Astronomy Observatories in Tucson, Arizona, cautions that more work is needed. In particular, spectroscopic analysis of the objects' emissions would determine their temperature, which could be used to confirm their mass. But Najita says the essential message is believable.

The objects don't neatly fit any conventional definitions. Brown dwarfs are usually smaller than about 75 Jupiter masses, the minimum size needed to ignite the hydrogen stars need to burn, but larger than 13 Jupiter masses, what's necessary to fuse deuterium and produce a faint glow. By failing to reach this lower limit, the new objects are hard to account for. Most astrophysicists believe brown dwarfs and stars condense directly out of vast seas of tenuous gas known as molecular clouds, whereas planets form in disks of matter swirling around nascent stars. Small lone bodies, however, don't mesh well with either scenario.

Two theories about the origins of planetary objects shed light on the elusive creations but fall short of supplying a complete answer. One proposes that they are ejected from young stellar systems, the other that they form from molecular cloud cores with masses too low to give birth to stars. But Inutsuka says neither idea can account for the large numbers of smaller objects spotted in S106. "I think [Oasa's report] will prove extremely important for pushing the modification of currently accepted theories of star formation," he says.